Zirconium metal has historically been a material of construction, in particular cladding for fuel rods, for nuclear reactors, and there has been a continuing interest in reducing its tendency to adsorb thermal neutrons. The more transparent the internal materials of construction of a nuclear reactor are to such thermal neutrons the more efficiently the reactor will function since a certain number of these neutrons are necessary to sustain the nuclear reaction and their production must compensate for the adsorption by the internal materials of construction. Early efforts were directed to separating zirconium from hafnium. The two elements occur together naturally but the hafnium has a substantially larger capture section for thermal neutrons. Such efforts involved both chromatographic techniques using an ion exchange resin and various solvent extraction techniques.
Recent efforts have been directed to isolating a zirconium isotope with either a particularly high or a particularly low cross section to thermal neutrons. This allows the production of a zirconium with a lower average cross section than one composed of the naturally occurring isotope distribution. These efforts at isotope separation have generally involved some type of solvent extraction. These separation techniques are generally only able to separate one isotope at a time. Thus they do not provide a means for simultaneously isolating the zirconium 90 and 94 isotopes which are recognized as having particularly small cross sections (one source lists them as 0.055 and 0.031 Barns, respectively, as compared to 0.567 Barns for zirconium 91 and 0.1430 for zirconium 92).
More recently, it has been proposed that isotopes of zirconium could be separated in an economically practical manner by the use of continuous steady state chromatography utilizing a cation exchange resin as the stationary phase. The preferred stationary phase in this proposal was sulfonated crosslinked polystyrene beads. It appears that this proposal provides a continuous process for isolating both of the abundant low thermal cross section isotopes, zirconium 90 and zirconium 91, in a single procedure. However, it is difficult to get the concentration of zirconium in the product of elution volumes as high as desired. Thus, larger than desired volumes of spent eluant must be dealt with and this poses a problem of waste management. This concentration can be increased by shortening the residence time on the chromatograph and this in turn can be achieved by increasing the separation capacity of a theoretical stage without adversely effecting the achievable flow rates through the column. Such an improvement in theoretical stage efficiency would also permit the same separations to be effected on shorter columns.
It is an object of the present invention to provide an improved process for chromatographically separating the isotopes of zirconium which conveniently yields improved concentrations of zirconium in the product elution volumes, and reduces the volume of waste liquor to be recycled or processed for disposal.
It is a further object of the present invention to provide a process which allows the use of shorter chromatographic columns thus reducing both the capital and operating costs of the process. It is an additional object of the present invention to provide a process which yields separate waste streams of heavy metal waste and radio chemical waste each with an increased solids content. Another object of the present invention is to provide a process which utilizes a cation exchange resin which yields a higher theoretical stage separation without impairing the flow rates achievable on the column.